‘Fat but fit’ people may still be at risk of heart disease

"Concept of being 'fit but fat' is a myth, researchers say," ITV News reports after a Europe-wide study looked at associations between body weight, metabolic health and heart disease.

The term "fat but fit" is used to describe people who are overweight or obese but don't have any of the symptoms of metabolic syndrome.

This is a common complication of obesity, and symptoms include high cholesterol, high blood pressure and an inability to control blood sugar levels (insulin resistance).

The study involved 17,640 people. Body weight was used to calculate each person's body mass index (BMI) and various tests were used to assess each person's metabolic health. They were then followed for 12 years to look at the development of heart disease.

The findings showed that BMI was an independent risk factor for heart disease, regardless of metabolic health.

Obese people who were metabolically healthy – the "fat but fit" – had a 28% higher heart disease risk than their normal weight, metabolically healthy counterparts.

But metabolic risk factors still seem to carry the highest risk for heart disease. People who were a normal weight but metabolically unhealthy had a more than doubled risk of heart disease. So despite being a normal weight, their risk was actually higher than the "fat but fit".

The good news is that certain lifestyle changes can prevent or reverse metabolic syndrome and reduce your risk of heart disease.

What kind of research was this?

This was an analysis of a random sample of people taking part in the large European Prospective Investigation into Cancer and Nutrition study (EPIC).

As the title suggests, EPIC is an ongoing cohort study looking at the links between diet and cancer. Because of the large amount of data the EPIC study gathers, researchers can also use the data to look at other associations by focusing on smaller groups (a sub-cohort).

Within this sub-cohort, known as EPIC-CVD (cardiovascular disease), the researchers compared cases of people who developed heart disease with those who didn't in a case-cohort study.

The aim was to investigate the theory of "metabolically healthy obesity". This is the idea that people who have excess body fat can still be healthy if they don't have other metabolic risk factors like high blood pressure, high cholesterol and poor blood sugar control.

To date, there's been conflicting evidence about whether the metabolically healthy obese have a higher or lower risk of heart disease and type 2 diabetes.

This study aimed to use the large body of data collected from the EPIC-CVD cohort to better look into this.

What did the research involve?

Between 1991 and 1999 EPIC recruited 366,521 women and 153,457 men aged 35-70 from 10 countries across Europe.

The sub-cohort for the EPIC-CVD project consisted of a random sample of 17,640 adults who were free from stroke or heart disease at baseline.

Participants completed questionnaires on medical history, diet and lifestyle at the start of the study.

They gave blood samples to measure total cholesterol and blood sugar, and their blood pressure, weight, height and waist circumference was also measured.

Being metabolically unhealthy or having metabolic syndrome was defined as having three or more of the following at baseline:

high blood pressure, use of blood pressure medications, or self-reported history

high triglycerides (a type of fat) or use of lipid-lowering medication like statins

low HDL (good) cholesterol

high blood sugar, use of diabetes medications, or self-reported history

high waist circumference

Researchers looked for the new development of heart disease during follow-up, either self-reported or through data from GP and hospital registers and mortality records. Last follow-up ranged from 2003-10, with an average of 12.2 years.

Adjusting for metabolic risk factors of blood pressure, cholesterol and diabetes substantially reduced the link with BMI, showing the importance of these factors. But the link was still statistically significant (HR 1.05, 95% CI 1.01 to 1.10).

Waist circumference was similarly associated with heart disease risk (HR 1.32, 95% CI 1.24 to 1.41). Again, adjusting for blood pressure, cholesterol and diabetes similarly reduced the link so it was on the threshold of significance (HR 1.06, 95% CI 1.00 to 1.13).

But metabolic health still seemed to be the most important factor. Normal weight people who were metabolically unhealthy had a more than doubled risk of heart disease than metabolically healthy normal weight people (HR 2.15, 95% CI 1.79 to 2.57).

Metabolically unhealthy obese people had an even higher risk compared with metabolically healthy people of a normal weight (HR 2.54, 95% CI 2.21 to 2.92).

They say that their findings "challenge the concept of 'metabolically healthy obesity', encouraging population-wide strategies to tackle obesity".

Conclusion

This large, valuable study confirms that – as has long been thought – an increased BMI is linked with an increased risk of heart disease.

It shows that people with an obese BMI had a higher risk of heart disease, even if they didn't have other risk factors like high cholesterol and high blood pressure, proving that body fat is an independent risk factor.

That said, this study does have some limitations. For example, definitions of being metabolically unhealthy aren't entirely consistent with other definitions of metabolic syndrome.

This was also only assessed at the start of the study, and risk factors may have varied among individuals during follow-up.

And not all participants in the centres across Europe were general population samples. They may have included a higher proportion of those with risk factors.

Nevertheless, the study on the whole supports our understanding of the modifiable risk factors for heart disease, and shows that being obese in itself increases your risk, regardless of your metabolic health.

More importantly, it also shows it's possible to be "not fat but unfit" if your BMI is within the recommended range but you have one or more metabolic risk factors.